The figure depicts selected aspects of a very-high-frequency (VHF) microstrip-patch antenna designed and built to satisfy requirements specific to an airborne synthetic-aperture radar system for measuring the thickness of sea ice. One of the requirements is that the antenna be capable of functioning over the relatively wide frequency band of 127 to 172 MHz — corresponding to a fractional bandwidth of about 30 percent relative to a nominal mid-band frequency of 149.5 MHz. Another requirement is that the antenna be capable of functioning in either or both of two orthogonal linear polarizations. In addition, the antenna is required to be as compact and lightweight as possible.
In a basic design according to generally accepted microstrip-patch-antenna-engineering practice, one would ordinarily use a relatively thick dielectric substrate and multiple feed probes to obtain the desired combination of wide-band and dual-polarization capabilities. However, the combination of a thick substrate and multiple feeds would give rise to higher-order electromagnetic nodes, thereby undesirably contributing to cross polarization and to reduction of the isolation between feed probes. To counter these adverse effects while satisfying the requirements stated above, the design of this antenna incorporates several improvements over the basic design.
The antenna features dual stacked square radiator patches, a ground plane, and relatively thick dielectric made of foam having a low value (1.05) of relative permittivity. The sides of the top and bottom radiator patches are 69.3 cm and 76.2 cm long, respectively. The patches are mechanically supported by the dielectric substrates. The bottom radiator patch lies 6.5 cm above the ground plane, which is a square of side length 117 cm. The top radiator patch lies 10.16 cm above the bottom radiator patch.
The bottom radiator patch is excited via square capacitive feed probes, the capacitive patches of which have a side length of 6.35 cm. The top radiator patch is excited parasitically from the bottom radiator patch. Four feed probes (instead of the minimum of two feed probes needed for dual polarization) are used to increase the wide-band capability, suppress higher-order modes, and reduce cross-polarization levels. Each probe is located 2.54 cm from the center of the antenna and its capacitive patch is located 1.4 cm below the bottom radiator patch. Within each pair of oppositely located feed probes, the two probes are excited 180° out of phase with each other to suppress higher-order modes. For additional suppression of higher-order modes, a shorting pin is soldered to both the upper and lower patches and the ground plane at the center of the antenna.
In the absence of corrective action, the use of four feed probes and thick substrates would result in unacceptably large amounts of coupling between oppositely located probes. To reduce this coupling, 24 additional shorting pins, located along the two axes of symmetry, are soldered between the bottom patch and the ground plane. Power is coupled to the antenna via two 180° hybrids, each for exciting one of the pairs of oppositely located probes.